Conventionally, the technology of using the light in the 1.31 μm or 1.55 μm wavelength band as signal light in the optical communications is well established. The optical fiber, which is mainly used as an optical transmission line in optical communication, is a standard Single Mode Fiber (SMF) having zero dispersion wavelength in the 1.31 μm band (1.31 μm band is defined as the wavelength range of 1300 nm-1324 nm, hereinafter).
Meanwhile, since the transmission loss of the silica based optical fiber becomes minimum in the 1.55 μm band and the Erbium Doped Fiber Amplifier (EDFA) can carry out high efficient optical amplification in the 1.55 μm band, the Dispersion Shifted Fiber (DSF) having zero dispersion wavelength in the 1.55 μm band has been widely introduced. (1.55 μm band is defined as the wavelength range of 1530 nm-1570 nm, hereinafter.)
The so-called dual shape profile consisting of a first core at the center and a second core having a refractive index lower than that of the first core surrounding the first core is generally deployed as the refractive index profile of the core of DSF.
In recent years, rapidly increasing the information traffic due to the advance of the information technology, WDM transmission has been widely introduced into the telecommunications field. The optical fiber for WDM transmission is required to have low transmission loss. Moreover, in order to prevent the noise generation by nonlinear phenomenon such as four-wave mixing (FWM) etc., it is also important that the absolute value of the dispersion should not be too small in the operating wavelength band.
Furthermore, in order to suppress the wave distortion of the signal light due to the accumulative dispersion, it is also important that the absolute value of the dispersion is not too high.
As one solution of the optical fiber to prevent the above-mentioned noise generation by FWM, a non-zero dispersion shifted fiber (NZ-DSF), in which the refractive index profile of the conventional DSF is adjusted and the zero dispersion wavelength slightly shifted to the longer or shorter side of 1550 nm, is developed and commercialized.
In addition to FWM, there are a self-phase modulation (SPM), a cross phase modulation (XPM), etc. in a nonlinear phenomenon which are known to be proportional to the optical power density in the core of the optical fiber.
Even if NZ-DSF could suppress the FWM, distortion of the signal waveform by SPM and XPM is generated easily. This is because the optical power density in the core easily increases since the mode field diameter (MFD) of NZ-DSF is comparatively small.
Although longer distance transmission with many wavelengths requires higher optical power input, the optical power density can be reduced by enlarging an effective area (Aeff), which can result in reducing the waveform disorder by the nonlinear phenomenon.
Therefore, the optical fiber having a higher effective area and a moderate dispersion in the operating wavelength range is preferable to suppress the nonlinear phenomenon.
The optical fiber in which the generation of the above-mentioned nonlinear phenomenon is suppressed and the distortion of the signal light caused by the nonlinear phenomenon decreased, by enlarging Aeff more than about 70 μm2, is disclosed in U.S. Pat. No. 6,072,929.
Meanwhile, further expansion of the communication information capacity is required, and in addition, the attempt to enhance the operating wavelength band is investigated. The U.S. Pat. No. 5,905,838 discloses the optical fiber that enables WDM transmission in both the 1.31 μm and 1.55 μm wavelength bands.
In the optical fiber disclosed by the U.S. Pat. No. 5,905,838, the zero dispersion wavelength was shifted into the range between 1350 nm and 1450 nm and the absolute value of dispersion is made small in both the 1.31 μm and 1.55 μm band by reducing the dispersion slope in both the wavelength bands.